Can someone explain what this line is doing?

Question

In this blog: http://www.reigndesign.com/blog/using-your-own-sqlite-database-in-android-applications/

This is one line of this.getReadableDatabase(); I don’t understand what she is doing, but if I remove it from my code, it will stop working.

 /** * Creates a empty database on the system and rewrites it with your own database. * */ public void createDataBase() throws IOException{ boolean dbExist = checkDataBase(); if(dbExist){ //do nothing - database already exist }else{ //By calling this method and empty database will be created into the default system path //of your application so we are gonna be able to overwrite that database with our database. this.getReadableDatabase(); try { copyDataBase(); } catch (IOException e) { throw new Error("Error copying database"); } } } 

Answer 1

From the implementation of the function, you can see that this api opens the database by calling getWritableDatabase (). In the event of a failure for some reason, it opens db in read-only mode.

 public synchronized SQLiteDatabase getReadableDatabase() { if (mDatabase != null && mDatabase.isOpen()) { return mDatabase; // The database is already open for business } if (mIsInitializing) { throw new IllegalStateException("getReadableDatabase called recursively"); } try { return getWritableDatabase(); } catch (SQLiteException e) { if (mName == null) throw e; // Can't open a temp database read-only! Log.e(TAG, "Couldn't open " + mName + " for writing (will try read-only):", e); } SQLiteDatabase db = null; try { mIsInitializing = true; String path = mContext.getDatabasePath(mName).getPath(); db = SQLiteDatabase.openDatabase(path, mFactory, SQLiteDatabase.OPEN_READONLY); if (db.getVersion() != mNewVersion) { throw new SQLiteException("Can't upgrade read-only database from version " + db.getVersion() + " to " + mNewVersion + ": " + path); } onOpen(db); Log.w(TAG, "Opened " + mName + " in read-only mode"); mDatabase = db; return mDatabase; } finally { mIsInitializing = false; if (db != null && db != mDatabase) db.close(); } } 

Here is the implementation of getWritableDatabase ()

  public synchronized SQLiteDatabase getWritableDatabase() { if (mDatabase != null && mDatabase.isOpen() && !mDatabase.isReadOnly()) { return mDatabase; // The database is already open for business } if (mIsInitializing) { throw new IllegalStateException("getWritableDatabase called recursively"); } // If we have a read-only database open, someone could be using it // (though they shouldn't), which would cause a lock to be held on // the file, and our attempts to open the database read-write would // fail waiting for the file lock. To prevent that, we acquire the // lock on the read-only database, which shuts out other users. boolean success = false; SQLiteDatabase db = null; if (mDatabase != null) mDatabase.lock(); try { mIsInitializing = true; if (mName == null) { db = SQLiteDatabase.create(null); } else { db = mContext.openOrCreateDatabase(mName, 0, mFactory); } int version = db.getVersion(); if (version != mNewVersion) { db.beginTransaction(); try { if (version == 0) { onCreate(db); } else { onUpgrade(db, version, mNewVersion); } db.setVersion(mNewVersion); db.setTransactionSuccessful(); } finally { db.endTransaction(); } } onOpen(db); success = true; return db; } finally { mIsInitializing = false; if (success) { if (mDatabase != null) { try { mDatabase.close(); } catch (Exception e) { } mDatabase.unlock(); } mDatabase = db; } else { if (mDatabase != null) mDatabase.unlock(); if (db != null) db.close(); } } } 

Answer 2

this.getReadableDatabase(); calls the getReadableDatabase () method; from the SQLiteOpenHelper class. This is possible because the DataBaseHelper is a subclass of SQLiteOpenHelper. This is determined by this line.

 public class DataBaseHelper extends SQLiteOpenHelper 

Answer 3

This class is a subclass of SQLiteOpenHelper .

Take the method from the link: [1]

Create and / or open a database that will be used for reading and writing. On the first call, the database will open onCreate (SQLiteDatabase), onUpgrade (SQLiteDatabase, int, int) and / or onOpen (SQLiteDatabase).

Therefore, it simply prepares SQLiteDatabase for you and processes the installation life cycle with the update, etc.

[1] http://developer.android.com/reference/android/database/sqlite/SQLiteOpenHelper.html#getReadableDatabase%28%29